Of the many hydrocarbon conversion processes known in the art, hydrocracking is becoming increasingly important since it offers product flexibility together with product quality. As it is also possible to subject rather heavy feedstocks to hydrocracking it will be clear that much attention has been devoted to the development of hydrocracking catalysts.
Whereas in the past catalytic hydrocracking was aimed primarily at the production of lower boiling point products such as gasoline, nowadays hydrocracking is often aimed at meeting the increasing demand for high quality middle distillate products.
Therefore, the object nowadays in hydrocracking is to provide a hydrocracking catalyst having a high selectivity towards middle distillates combined with a low selectivity towards gas make, in addition to high activity and stability.
To this end modern hydrocracking catalysts are generally based on zeolitic materials which may have been adapted by techniques like ammonium-ion exchange and various forms of calcination in order to improve the performance of the hydrocracking catalysts based on such zeolitic materials.
One of the zeolites which is considered to be a good starting material for the manufacture of hydrocracking catalysts is the well-known synthetic zeolite Y as described in U.S. Pat. No. 3,130,007. A number of modifications has been reported for this material which include, inter alia, ultrastable zeolite Y (U.S. Pat. No. 3,536,605) and ultrahydrophobic zeolite Y (GB-A-2,014,970) which are hereby incorporated by reference.
It is known that the stability and activity of an ultrastable zeolite Y (USY) can be improved further by subjecting it to a high-temperature steaming step followed by a dillumination step. In this respect reference is made, for instance, to GB-A-2,114,594. It is thought that such a high-temperature steam treatment causes some tetrahedrally coordinated aluminum (Al) species to be removed from the crystal framework. The holes formed in the crystal framework as a result of the removal of these tetrahedrally coordinated aluminum (Al) species are believed to be filled by silicon (Si) species. The aluminum (Al) species which are removed from the crystal framework are present in the pores and channels of the crystal framework and are mainly octahedrally coordinated. These octahedrally coordinated aluminum (Al) species can be removed in the subsequent dealumination step. To this end the steamed material is usually leached with an organic chelating agent, an organic acid or an inorganic acid.
Since the presence of octahedrally coordinated aluminum (Al) species in the pores and channels of the crystal framework is believed to reduce accessibility to the active sites of the crystal framework, it is very desirable if zeolite Y materials can be prepared containing little or no octahedrally coordinated aluminum (Al). Whilst the above dealumination treatment is effective in removing octahedrally coordinated aluminum (Al) species from ultrastable zeolite Y (USY) materials, the results obtained are unsatisfactory when it is applied to very ultrastable zeolite Y (VUSY) materials.